The invention relates to an electrical appliance having a lighting device having light emitting diodes.
This kind of appliance is, for example, a flat-screen television having a liquid crystal display and a light emitting diode backlight.
FIG. 1 shows a block diagram of such a television according to the prior art.
The television 1 has a mains connection 7 to supply power via the main power grid. A voltage supply 2 having a power factor correction circuit (PFC) 3 is disposed at the mains connection 7. Connected to the output of the PFC 3 is a DC/DC converter 4 that provides several secondary voltages 5 for various subsystems 6 of the television 1. The backlight 8 is supplied via one of these secondary voltages 5.
Connected downstream of the PFC 3, there is moreover an additional voltage converter 9 by means of which a circuit 10 to monitor and control a standby mode of the television 1 is supplied. The backlight 8 consists of a plurality of light emitting diode strings 11 in which a plurality of light emitting diodes (LED) 12 are disposed. Since individual light emitting diodes 12 are subject to series fluctuations, at a constant voltage, the same current does not flow into each string 11, thus also causing a deviation in brightness. To ensure homogeneous brightness for the backlight 8 over the entire surface, the current in the individual strings has to be adjusted.
Hence each string 11 has an additional DC/DC converter 13 by means of which the DC voltage for each string 11 can be separately regulated. This separate voltage control is mainly used to achieve current balancing of the individual strings 11. In addition, however, the brightness of the individual strings 11 has to be separately regulated in order to dim the backlight 8, since other consumer loads 6 are also connected to the main voltage converter 4.
Since it is necessary to have a separately adjustable voltage converter in each string, the circuit is made complex and expensive.
In addition, the overall efficiency of the voltage supply is impaired by the additional voltage converters of the individual strings.